1. Field of the Invention
The present invention relates to an antenna apparatus for a thin and small wireless device, for example, and more particularly, to a technique for arranging an antenna on a high-impedance substrate.
2. Related Art
An Electromagnetic Band Gap (EBG) substrate is known as a technique for arranging a metallic plate (or a ground plane) and an antenna in close proximity to each other for the purpose of making an antenna apparatus thin. An EBG substrate is structured by arranging planar elements in a matrix at a certain height over a metallic plate and connecting the planar elements with the metallic plate via linear elements. The EBG substrate realizes high impedance by creating LC parallel resonance circuits by way of distributed circuits and suppresses unnecessary current distribution that can be generated on the metallic plate.
However, since current locally distributes also on the EBG substrate, degradation of antenna characteristics occurs when the EBG substrate and the antenna are arranged very closely to each other. This is because current distribution on the antenna significantly varies due to an effect of current distributed on the EBG substrate, resulting in impossibility of matching. Meanwhile, a monopole antenna encounters a problem of an inability to make effective use of radiation from the ground plane, which is a characteristic of the monopole antenna, because current on the ground plane is suppressed.
Due to these facts, EBG substrates generally suppress degradation of antenna characteristics resulting from mutual coupling by not positioning the antenna and the EBG substrate very closely to each other. However, such a method imposes a limit on reducing the thickness of an antenna apparatus.